1. Field of the Invention
The present invention relates to illumination optical systems. An illumination optical system of the present invention is suitable for illumination optical systems such as those included in exposure apparatuses using exposure light of extreme ultraviolet (EUV) having a wavelength of 10 nm to 15 nm.
2. Description of the Related Art
From the viewpoint of improving the resolution, the wavelength of exposure light used in semiconductor exposure apparatuses has been becoming shorter over time. Examples of next-generation exposure apparatuses using a light source having a shorter wavelength include EUV exposure apparatuses, which uses light of the extreme ultraviolet region (EUV light) whose wavelength ranges from 10 nm to 15 nm.
Light of the extreme ultraviolet region is largely absorbed by substances, and is therefore not practical for use with dioptric systems constituted mainly by lenses. Instead, catoptric systems are used in EUV exposure apparatuses.
An exemplary illumination optical system for EUV exposure apparatuses that include catoptric systems is disclosed in Japanese Patent Laid-Open No. 2005-141158 (corresponding US Patent: US2005/105290A1). In this illumination optical system, an aperture stop having a semicircular opening is disposed on a reflective integrator. The reflective integrator disclosed in Japanese Patent Laid-Open No. 2005-141158 is an optical integrator in which a plurality of cylindrical reflection surfaces are aligned with the generating lines thereof being parallel to each other.
In exposure apparatuses, to realize formation of high-quality images with a projection optical system, it is necessary to suppress distortion in the effective light source shape, which is observed from arbitrary points on a surface to be illuminated, in an illumination optical system.
It was considered that the illumination optical system disclosed in Japanese Patent Laid-Open No. 2005-141158 realized sufficient suppression of distortion in the effective light source shape compared to known examples at the time of disclosure. However, as a result of reviews by the present inventors, it has been found that the illumination optical system of Japanese Patent Laid-Open No. 2005-141158 still has some distortion in the effective light source shape and that the degree of this distortion is not negligible. The reason for the remaining distortion in the effective light source shape of the illumination optical system disclosed in Japanese Patent Laid-Open No. 2005-141158 is because part of parallel light is regulated by the aperture stop before forming a secondary light source.
This phenomenon will be described in detail with reference to FIGS. 22 and 23. FIG. 22 is a schematic perspective view showing the arrangement of a reflective integrator 13 and an aperture stop 15 in Japanese Patent Laid-Open No. 2005-141158. FIG. 23 is a top view of the reflective integrator 13 shown in FIG. 22.
In FIG. 23, an output-side portion of a region 20 (FIGS. 22-23) (the hatched part) on the reflective integrator 13 illuminated with illuminating light IL (FIGS. 22-23) is made smaller because of the regulation of the aperture stop 15. To obtain an effective light source shape of a perfect circle at a point 31 on an end of an arcuate region 30 that is to be illuminated, a region 21 enclosed by a broken line is illuminated with the illuminating light IL.
The region 21 overlaps the region 20 on the incident side with respect to the aperture stop 15 (the lower half in FIG. 23). This means that a region that is illuminated with light to obtain an effective light source shape of a perfect circle is ensured. The excessive light outside the region 21 is regulated by the aperture stop 15. That is, in FIG. 23, the lower half of the region 21 with respect to the aperture stop 15 corresponds to the lower half of an effective light source 32, which is a semicircle having no distortion.
In contrast, on the output side with respect to the aperture stop 15 (the upper half in the top view shown in FIG. 23), there is a portion where the region 21 and the region 20 do not overlap. This causes a shift (distortion) of the effective light source 32 from the perfect circle. The portion of the region 21 that is not illuminated with light makes the effective light source 32 to be eclipsed from the perfect circle. The excessive light outside the region 21, which is free from regulation without having any stops, corresponds to a portion of the effective light source 32 outside the perfect circle.
As described above, since the illumination optical system disclosed in Japanese Patent Laid-Open No. 2005-141158 still has a slight distortion in the effective light source shape, it has become impossible for this system to accommodate recent demands for higher image-forming performance.